firewall-util: reject NULL source or address with prefixlen 0

firewall-util: reject NULL source or address with prefixlen 0

Make sure we don't add masquerading rules without a explicitly
specified network range we should be masquerading for.

The only caller aside from test case is
networkd-address.c which never passes a NULL source.

As it also passes the network prefix, that should always be > 0 as well.

This causes expected test failure:
Failed to modify firewall: Invalid argument
Failed to modify firewall: Invalid argument
Failed to modify firewall: Invalid argument
Failed to modify firewall: Protocol not available
Failed to modify firewall: Protocol not available
Failed to modify firewall: Protocol not available
Failed to modify firewall: Protocol not available

The failing test cases are amended to expect failure on
NULL source or prefix instead of success.